1. Field of the Invention
This invention relates to high-power microwave/mm-wave generators, and more particularly to oscillators which operate by coupling an electron beam to a slow electromagnetic wave in a plasma-loaded, rippled-wall waveguide.
2. Description of the Related Art
Several devices are known which function as high power microwave or mm-wave generators, such as virtualcathode oscillators (vircators), magnetrons, klystrons, gyrotrons, and backward-wave oscillators. Such devices are described in J. Feinstein and K. Felch, "Status Review of Research on Millimeter-Wave Tubes", IEEE Transactions on Electron Devices, Vol. ED-34, No. 2, February 1987, pp. 461-467; H. K. Florig, "The Future Battlefield: A Blast of Gigawatts?", IEEE Spectrum, March 1988, pp. 50-54; Gordon T. Leifeste et 11., "Ku-Band Radiation Produced by a Relativistic Backward Wave Oscillator", J. Appl. Phys., 59(4), Feb. 15, 1986, pp. 1366-1378; and James Benford, "High Power Microwave Simulator Development", Microwave Journal, December 1987, pp. 97-105. With numerous variations, the approach generally is to couple an electron beam with an evacuated waveguide structure at a high vacuum, on the order of 10.sup.-6 Torr or less. A space-charge wave is induced on the electron beam and couples within the waveguide structure to an electromagnetic waveguide mode, and thereby emits microwave or mm-wave energy at the end of the guide.
Several limitations and disadvantages have been encountered with this approach. A high, or "hard", vacuum can be difficult to maintain at ultra high power levels. Also, the electrons in the beam establish a mutually repulsive space-charge, which without a controlling mechanism causes the beam to rapidly expand and destroy any beam focusing or collimation; this is referred to as space-charge blowup. As a consequence, a very strong magnetic field of up to 10 kGauss must be employed to confine the beam, which complicates the structure, reduces efficiency and adds to the expense of the microwave generator. Even when a magnetic field is used, a potential depression still occurs across the beam, and the negative potential reduces the beam voltage in the vicinity of its axis. The result is that the electrons slow down near the beam axis, a phenomenon referred to as axial velocity shear, which impedes the achievement of good coupling between the beam and the waveguide structure.
At very high output powers the prior devices cannot generate pulse lengths longer than a few hundred nanoseconds because they use field-emitting cathodes in their electron guns; these generate an expanding uncontrolled plasma surface in the evacuated high-voltage-diode electron gun gap. The plasma surface propagates from cathode to anode, shorting the gap in 100-1,000 nanoseconds, and thus terminating the pulse. Devices such as the vircator also use a metal-foil anode that self-destructs in about 100 nanoseconds.
The magnetic focusing required to counteract space-charge blowup needs a very strong magnetic field, on the order of 10 kGauss or more, and associated bulky magnets. The axial velocity shear produced by the space-charged fields also reduces the efficiency of the oscillator at high beam current densities.
Other types of electron guns include plasma anode devices, and wire ion plasma guns. The former device is described in U.S. Pat. No. 4,707,637 issued Nov. 17, 1987 in the name of Robin J. Harvey, while the latter is described in U.S. Pat. No. 4,025,818, issued May 24, 1977 in the name of Robert P. Giguere, both assigned to Hughes Aircraft Company, the assignee of the present invention. Another electron gun is described in U.S. Pat. No. 3,831,052, issued Aug. 20, 1974 in the name of Ronald C. Knechtli, and also assigned to Hughes Aircraft Company. The latter device is a hollow cathode gas discharge mechanism used to produce an electron beam with a rectangular cross-section for driving gas lasers. Current densities in the range of 10.sup.-4 to 1 amp/cm.sup.2 are described. A discharge is struck through a gas within the cathode, between the cathode walls and a rectangular perforated anode which is situated within a cathode exit slit. A relative positive polarity is applied to the anode electrode to extract electrons from the plasma. The electrons are accelerated by a greater positive polarity on a control grid, and once past the control grid are further accelerated by a high voltage accelerating field between a thin foil window and the grid.